Etching bath



United States Patent ETCHING BATH George Schwan, Glen Head, N.Y., assignor to Powers ghelmco, lnc., Glen Cove, N.Y., a corporation of New No Drawing. Application April 23, 1956 Serial No. 579,714

6 Claims. (CI. 41-42) The present invention relates to a novel and improved process and bath or composition of matter for the etching by nitric acid of metals, such as magnesium and zinc.

The present invention in many respects is an improvement on the Dow etching process which is widely used for the etching of magnesium and zinc metals, such as for the formation of a relief surface, and is especially adapted to the making of printing plates. The Dow etching process is fully described in the prior patents to Easley and Swayze Nos. 2,640,763, 2,640,764, 2,640,765, 2,640,766 and 2,640,767, granted June 2, 1953, to which reference is made for a full and detailed description of the process and the compositions of matter usable in said process of the prior art.

The present invention has for its object the provision of a novel and improved etching bath and process for .the etching of metals, especially magnesium and zinc,

which is simpler in use, more stable and more satisfactory than the baths of the prior art. A further object is the provision of such-a process and bath which does not require repeated adjustment of its composition during its useful life.

According to the improvement of the present invention, the etching process employs a nitric acid etching bath in accordance with the prior patents above-referred to, but with the addition thereto of a single small quantity of polyvinyl pyrrolidone in place of the continuous addition of small amounts of gelatine which is normally used in the commercial practice of the Dow etching process.

It will be understood that the foregoing general description and the following detailed description as well are exemplary and explanatory of the invention but are not restrictive thereof.

The etching baths to which the improvements of the present invention may be adapted are comprised of an aqueous solution of nitric acid containing from 1 to 50% by weight of nitric acid, a relatively small amount of an agent comprising one or more substances which increase the etch factor as defined by Easley and Swayze. Such agents which increase the etch factor are relatively insoluble organic materials which are liquid at moderately elevated temperatures and are substantially unreactive with the aqueous nitric acid solution. The agents which increase the etch factor are disclosed in detail by Easley and Swayze as comprising: the aliphatic acids having from 5 to 26 carbon atoms with no OH group other than in the carboxylic group, esters of aliphatic acids having from 12 to 18 carbon atoms reacted with polyhydric alcohols having from 2 to 6'carbon atoms; an

ester of sulfosuccinic acid and an aliphatic alcohol having from 6 to 12 carbon atoms and used in connection with a water-immiscible organic material which is liquid when used, boiling from 90 to 390 C. and unreactive with the nitric acid solution, such as a petroleumrfraction, turpentine, perchloroethylene or diethylbenzene; the ester of succinic acid with a polyhydric alcohol having bi'iUbb 2,890,943 Patented June 16, 1959 "ice from 6 to 12 carbon atoms used alone but in larger quantities than when used with the petroleum fraction; and petroleum sulfonic acid soaps, such as naphthenic soap or mahogany soap, used with a petroleum fraction boiling at from to 390 C.

In actual commercial practice of the Dow etching process, it has been found necessary for extended operation of the etching bath to add to it continuously during the etching operation a small quantity of a gelatinous material, preferably an aqueous solution of gelatin which is ordinarily added at the rate of about 1 drop per second of a 5% solution of gelatine to a liter bath, or at the rate of 1 part per second per 2,000,000 parts of bath. The exact rate of gelatin addition depends upon actual operating conditions, and considerable difficulty is often encountered in establishing the correct rate of gelatin addition, a factor which is further complicated when the bath is used intermittently. Due to the rapid degradation of the gelatin by the nitric acid, it is not possible to add gelatin except as a constant addition at a relatively slow rate, such as the 1 drop per second referred to. Such additions of gelatin are not needed except when plates are being etched, but continued additions during the periods when the bath is not being used would soon contaminate the bath with the degradation products of the gelatin. Also, when starting up a bath, or when resuming operation of the bath after a period of a few hours, it is usually necessary to start the gelatin addition prior to the use of the bath, thereby slowing down the throughput of the work and requiring a considerable degree of judgment on the part of the operator.

The etching bath of the present invention comprises a mixture of an aqueous solution containing nitric acid, a relatively water-insoluble, etch-factor-increasing organic compound, substantially unreactive with the nitric acid and liquid at moderately elevated temperatures as used in operation of the bath, and from 2 to 400 and preferably 20 to 40 parts per million parts by weight of bath of polyvinyl pyrrolidone having a K value of from about 20 to 90. The K value of K-30 is determined according to the procedure of Finkentscher, Celluloschemie, vol. 13 (1932), pages 58 to 64 and 71 to 74. See also Miller and Hamm, Macromolecular Properties of Polyvinyl Pyrrolidone: Molecular Weight Distribution, J. P. Chem. 57, (1953). The values K-40, K-60 and K-90 indicate higher viscosities and K-20 a lower viscosity with higher and lower average molecular weights, respectively.

Polyvinyl pyrrolidone has the general formula:

where n approximates the average number of vinylpyrrolidone groups which are polymerized to form the polyvinylpyrrolidone.

Conveniently, the polyvinyl pyrrolidone is added in the form of a 5% aqueous solution, and for a bath of liters from about 50 to about 100 cc. of such a solution are added and thoroughly mixed, for optimum results.

As distinct from the continuous addition required when gelatin, glue or other proteinaceous material is added, only one addition of the polyvinyl pyrrolidone is required for the life of the bath.

The concentration of the polyvinyl pyrrolidone solution may be varied within wide limits with a corresponding adjustment of the quantity added so that the ultimate nitric acid bath includes from 2 to 400 parts per million by weight of the polyvinyl pyrrolidone, and most preferably from about 20 to 40 parts per million whether added Uti it i l N ti'i tities by weight:

Nitric acid 1% to 50%, preferably 10%.

Etch factor increasing agentpreferably diethyl benzene (preferably o o and 25% each meta and para diethyl benzene) 1% to 14%, preferably Sodium salt of diethyl sulfo succlmc a 1 0.1% to 2%, preferably 116%. Polyvinyl pyrrolidone K-30 20 to 40 parts per million.

Water, sufiicient to make 100%.

Other specific working examples comprise the baths disclosed in the several specific working examples of the Easley and Swayze' patents referred to with the addition thereto of from 2 to 400 parts per million of one of the polyvinyl pyrrolidones, such as the K-30 grade and less preferably the K-20, K-40, K-60 or K-90 grade, and most preferably the K-30 viscosity grade.

While the diethyl benzene combined with the sodium salt of diethyl sulfo succinic acid is the preferable etch factor increasing agent, good results are obtained by using any of the baths described in the Easley and Swayze patents according to which the etch factor increasing agent may comprise a relatively insoluble saturated aliphatic 'acid which is liquid at the moderately elevated temperatures of use, the esters of aliphatic acids having from 12 to 18 carbon atoms and polyhydric acids which have from 2 to 6 carbon atoms, an aliphatic alcohol ester ofdiethyl sulfosuccinic acid used with a water immiscible organic material which is liquid at the temperature of use as is substantially unreactive with nitric acid (a petroleum fraction boiling between 90 and 390 C., turpentine, perchloroethylene or o-, mor pdiethylbenzene), a wetting agent used alone or in combination with a silicone defoamer, or a petroleum sulfonic acid having from 18 to 30 carbon atoms, such as naphthenic soap or mahogany soap in combination with a petroleum fraction boiling from 90 to 390 C.. The useful ranges of such '4 etch factor increasing agents are given in the Easley and Swayze patents, together with the useful ranges of the nitric acid concentration, and the amount of polyvinyl pyrrolidone to be used is substantially independent of the exact composition of the etching bath.

The invention in its broader aspects is not limited to the specific steps, process and compositions shown and described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. An etching bath for etching an acid soluble metal comprising a mixture of an aqueous solution containing from 1 to 50 percent nitric acid, an etch factor increasing agent and from 2 to 400 parts per million by weight of polyvinyl pyrrolidone.

2. An etching bath as claimed in claim 1 in which the etch factor increasing agent is used in proportion from 20 to 40 parts per million on the weight of the etching bath.

3. An etching bath as claimed in claim 1 in which the etch increasing factor is a wetting agent and diethylbenzene.

4. An etching bath for etching an acid soluble metal comprising the following ingredients in substantially the following quantities, by weight:

Nitric acid About 10%. Diethyl benzene About 7%. Sodium salt of diethyl sulfosuccinic acid About 1 .6%. Polyvinyl pyrrolidone K-30 20 to 40 parts per million.

The remainder being substantially all water.

5. The process of etching an acid soluble metal which comprises initially adding from 2 to 400 parts per million by weight of polyvinyl pyrrolidone to an aqueous bath containing from 1 to 50 percent nitric acid and an etch-increasing factor, and immersing in said bath successive metal objects to be etched.

6. The process of claim 5 in which the polyvinylpyrrolidone is the K-30 grade and is used from 20 to 40 parts per million in the bath.

References Cited in the file of this patent UNITED STATES PATENTS 2,495,918 Keiser Ian. 31, 1950 2,640,765 Easley et al. June 2, 1953 2,658,045 Schildknecht Nov. 3, 1953 

1. AN ETCHING BATH FOR ETCHING AN ACID SOLUBLE METAL COMPRISING A MIXTURE OF AN AQUEOUS SOLUTION CONTAINING FROM 1 TO 50 PERCETN NITRIC ACID, AN ETCH FACTOR INCREASING AGENT AND FROM 2 TO 400 PARTS PER MILLION BY WEIGHT OF POLYVINYL PYRROLIDONE. 